Water Oxidation in Medium-Entropy Spinel Oxides via Lattice Oxygen Activation

The development of efficient oxygen evolution reaction (OER) catalysts requires advancements in both the mechanism understanding and material design. The lattice oxygen oxidation mechanism (LOM) typically has a lower thermodynamic barrier than the absorbate evolution mechanism (AEM), yet controlling the OER pathway from the AEM to the LOM remains challenging. Here, we demonstrate efficient lattice oxygen activation in a spinel-structured CoFeMoRu medium-entropy oxide (CoFeMoRuMEO) catalyst through strategic octahedral engineering. The introduction of Mo increases the electron density at the Co sites, thereby weakening OH adsorption and suppressing CoOOH formation via the AEM pathway. Meanwhile, compressed RuO6 octahedra create shortened Ru–O bonds, enhancing Ru–O covalency and facilitating the critical O–O coupling step. As a result, the CoFeMoRuMEO catalyst achieves a remarkable overpotential of 168 mV at 10 mA cm–2, setting a new benchmark for medium-to-high-entropy OER catalysts. Our work provides valuable insights into the transformation of the OER mechanism and performance optimization.